Polymerizing olefins in multizone circulating reactors may allow for the production of olefin polymers with a high productivity per unit volume of the reactor and with a high flexibility in the composition of the resulting polymers. Such multizone reactors are, for example, described in WO 97/04015 A1 and WO 00/02929 A1. WO 2005/019280 A1 describes polymerization processes comprising a combination of at least two gas-phase reactors, where at least one of the gas-phase reactors is a multizone circulating reactor. Multizone circulating reactors are gas-phase polymerization reactors in which the growing polymer particles flow upward through a first polymerization zone (riser), leave the riser and enter a second polymerization zone (downcomer) through which the growing polymer particles flow downward in a densified form. The densified polymer particles then leave the down-comer and are reintroduced into the riser. In this manner, a circulation of polymer between the riser and the downcomer is established. The gas employed for transporting the polymer particles in the riser may be separated from the polymer particles in the upper part of the multizone circulating reactor; recycled through a recycle gas line equipped with a compressor and a cooling unit for removing the heat of polymerization; and then re-used as transporting gas in the riser.
The industrial production of polyolefins may be carried out in continuous operation. That means, the input materials such as monomers, catalysts and polymerization additives are continuously introduced into the polymerization reactor and the obtained polyolefins are continuously withdrawn from the polymerization reactor. When it is desired to produce different polymer grades, the polymerization may be transitioned from one grade to another. In alternative embodiments, the transition may be done with minor (or no) interruption to polymerization. In order to be able to carry out certain maintenance or cleaning operations, it may nonetheless sometimes be needed to stop the polymerization reaction.
For re-starting (otherwise called “start up”) the polymerization in gas-phase polymerization reactors such as fluidized-bed reactors, a pre-prepared “seed bed” or “charge powder” of polyolefin particles may be first introduced into the reactor. Polymerization may be initiated after such introduction in the presence of such a bed. Another option is to start up the gas-phase polymerization with an empty reactor, i.e. with a reactor that does not contain any polymer particles. This implies that no facilities for storing seed bed polymers need to be provided. However, when starting a gas-phase reactor with an empty reactor, care may be taken such that the small particle size catalyst particles are not entrained with the recycle gas into the recycle gas system. The system may also be monitored to ensure that the heat of polymerization is removed in order to avoid temperature raise with subsequent chunk formation.